This invention relates to DNA polymerases, and in particular those having a processivity greater than that of DNA polymerase I of Escherichia coli.
Processivity is a measurement of the ability of a DNA polymerase to incorporate one or more deoxynucleotides into a primer template molecule without the DNA polymerase dissociating from that molecule. DNA polymerases having low processivity, such as the Klenow fragment of DNA polymerase I of E. coli, will dissociate after about 5-40 nucleotides are incorporated on average. Other polymerases, such as T7 DNA polymerase in the presence of thioredoxin, are able to incorporate many thousands of nucleotides prior to dissociating. In the absence of thioredoxin such a T7 DNA polymerase has a much lower processivity. Such processivity can be measured much as described by Tabor et al., JBC 262, 16212 (1987) and is thought to be advantageous in certain biochemical reactions such as DNA sequencing (see, Tabor U.S. Pat. No. 4,795,699).
As stated above, the T7 DNA polymerase (T7 gene 5 protein) by itself has low processivity. T7 gene 5 protein, however, binds tightly to a processivity factor, the host-encoded thioredoxin, in a one-to-one ratio. Thioredoxin stabilizes the binding of T7 DNA polymerase specifically to a primer-template, and the complex of the two proteins is highly processive, and is capable of extending a primer many hundreds of nucleotides without dissociating (Tabor et al, supra; Huber et al., JBC 262, 16224 (1987)).